Introduction to Physical Science
At Home Lab: Determining the Percent of Oxygen in Air
Upon Completion of this lab, students will:
? Determine the percentage of the air that is composed of oxygen
? Understand the creation of a vacuum effect
? Describe the equalization of atmospheric pressure inside and outside of the container
To Complete this lab, Students need:
? A small candle (emergency candles work well). Before you start experiment make sure that
when the candle is covered by the jar, there is at least 2 inches between the top of the candle
and the jar.
? Food coloring to see level of water.
? A shallow pan/plate with small amount of water, about ½ inch.
? A glass jar, it can also be shot glass, but it must have straight walls. Flask with green solution
would not work well.
? Masking Tape (a sharpie can also be used, but it will ruin your glass)
? A ruler that can read in millimeters (mm)
Introduction:
Air is a mixture of gases, primarily nitrogen - about 79%, oxygen - about 20%, and the rest of it is a
mixture of trace amounts of other gases such as carbon dioxide, argon, water vapor and more. Burning
(combustion) is chemical reaction in which one of the reactants is oxygen. During this experiment we
will burn a candle in a closed jar to determine the percent of oxygen in the jar, which will be the percent
of oxygen in the air.
Safety:
Since you will be using flames, take all necessary precautions. Tie back long hair, do not wear top with
loose sleeves. Do not leave flame unattended. Do not put hot glass on cold surface. If flame gets too
large put the jar over or extinguish it with water.
Procedure:
1. Attach the candle to the bottom of the shallow pan. A few drops of hot wax work nicely. You can
heat the bottom of the candle to make it stick. (You don't want the candle to tip over in the
water.)
2. Place a piece of masking tape on the side of the jar from the top to the bottom. This will be used
to mark water levels during the experiment.
3. Fill the pan up to about halfway with water and color the water with a few drops of food
coloring.
4. Place the jar over the UNLIT candle and mark the initial height of the water on the masking tape.
5. Mark a line on the tape that measures the bottom of the glass (if the glass were standing
upright).

6. Remove the jar and light the candle. Cover it quickly with the jar (do not let the jar move or
effect the candle or flame). Observe. Let the candle burn until the flame goes out. The level of
water in the jar will increase.
Trial Initial height of water (mm) Final height ofwater (mm) Difference in height (mm) Total Glass Height(mm)
1
2
3
4
Average (hDifference) (hTotal)

7. Why did the flame go out?
8. If the flame used up the oxygen in the jar, what would happen to the atmospheric pressure in
the jar?

9. Why would the change in atmospheric pressure lead to a rise in the water level in the jar?

10. Mark the final height of the water on the masking tape while jar is still in water.
11. Repeat steps 4-6 three more times.
12. Subtract the initial height from the final height for each and add that to the difference in height
column (hdifference).
13. Subtract the initial height from the bottom of the glass height to determine the initial height of
the air column, which is the total glass height (hTotal).
14. Average the difference of heights. Enter this number into the data table "Average". Do the
same for the total glass heights.
15. The amount of air in the jar can be thought of in terms of the volume of air in the cylinder. The
volume of a cylinder is calculated using the equation ????= ????????2?, where r is the radius of the jar,
and h the height of the air column in the jar. When the water level is raised the radius doesn't
change, but the height does, and so the volume does (this is because, without the oxygen, there
is less air in the jar). We will use this idea to calculate the percent of oxygen in air.
???????????????????????????????? ???????? ????????????????????????= ???????????????????????????????????????? ???????? ???????????????????????? ???????? ????????????
???????????????????????????? ???????????????????????? ???????? ???????????? = ????????2?????????????????????????????????????????
????????2?????????????????????
= ?????????????????????????????????????????
?????????????????????
% ????????????????????????= ???????????????????????????????? ???????? ????????????????????????∗100%
Calculate the percent of oxygen you found to be in the air.
16. The actual percent of oxygen in air is about 20%. What is the percent error in your
measurement?

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